My dd was 3lb 9oz at birth, when she was tiny she wore premme tots bots nappies, the lady who runs http://www.greenjellyshop.co.uk/
had little girl who was about the same size and we borrowed hers. Both Sharon's girls were dinky at birth, so she is an expert on nappies for tinies. She can source the premature nappies from them from Tots Bots. When Lois got a bit bigger she wore XS sandies, thirsties,xs fuzzi bunz, diddy diapers, bameasies, bimbles, kissaluvs, some ancient mothercare shaped nappies and nippa'd muslins.
Hope all goes well with the birth and they don't turn out too tiny.